In a series of mutation experiments, using both x-rays and EMS, an extensive cytogenetic analysis is being made of both mutant and nomutant treated chromosomes. The contribution of chromosomal rearragement to lethality, sterility, and visible mutation is being assessed. The distribution of lethal mutations along the X chromosome is being determined by tests with several different short duplications that can be used to "cover" induced lethal mutants. Particular regions that appear to be free of induced lethal effects, or to mutate at an especially low rate, are being explored in hope of finding evidence for the existence of persisting duplications in the normal genome. Two or three such cases are now known to exist, as we have recently found. Further, the difference between mutant and nonmutant rearrangement breakpoints is being investigated; is it a matter of exactly where, or how the chromosome is broken? Also, the detailed relationships of specific genes to particular salivary chromosome bands is being studied, as is the mutability of genes associated with large bands in comparison with that of genes associated with thin, delicate bands. Complementation testing is being carried out in tests of the one gene-one band hypothesis in several different regions of the X chromosome. In all experiments, the mutational consequences of treating germ cells at different stages of maturity is being compared (for both chromosomal and nonchromosomal mutants) by testing, separately, each successive brood of progeny produced after treatment.